Although organic polymers have replaced metals in many structural contexts, thus far they have failed to replace metals when the latter are used as electrical conductors or semiconductors. The impetus for such replacement includes, among others, lower cost, lower weight of materials, and increased processing variability for polymers as compared with metals. For example, polymers readily can be cast as films, foils, and fibers by standard, time-tested procedures. Polymers can be formed into a limitless variety of shapes and dimensions by standard processing procedures, thereby adding to the potential benefit of electrically conducting polymers.
A potential use for electrically conducting polymers is as electrodes or components of batteries, where their low weight and possibly unlimited scope of design are attractive. Electrically conducting polymers also could find use in construction of solar cells. Where such polymers are photoconducting they would undoubtedly find applications in the electrophotographic industry.
The conductivity ranges characterizing insulators, semi-conductors, and metallic conductors are somewhat arbitrary, but for convenience we may say an insulator has a conductivity less than about 10.sup.-10 ohm.sup.-1 cm.sup.-1, a conducting metal has a conductivity greater than about 10.sup.2 ohm.sup.-1 cm.sup.-1, and a semiconductor has a conductivity between the above. Quite often organic polymers which are insulators show a sufficient increase in conducting upon doping to act as semiconductors. By "doping" is meant adding a compound, referred to as a dopant, to the polymer so as to form a redox system wherein an electron is transferred from the polymer to the dopant, or vice versa. Two common examples of dopants are iodine (an electron acceptor) and sodium naphthalide (an electron donor). When the polymer transfers an electron to the dopant to exhibit semiconductor properties it is called a p-type semiconductor because conduction occurs mainly via holes in the valence band. Conversely, when the polymer accepts an electron from the dopant to exhibit semiconductor properties it is called an n-type semiconductor because conduction occurs mainly via electrons in the conduction band.
It is desirable for a normally insulating polymer to become a semiconductor upon doping by both p- and n-type dopants. It is also desirable that the polymer respond to a wide variety of dopants, and for its conductivity to be relatively responsive to changed levels of dopant. It is also desirable that the conductivity properties of the polymer remain stable over time and upon air exposure of the polymer. It is also quite desirable that upon doping the polymer remain flexible rather than becoming brittle.
It is a discovery of this invention that poly(nitrilo-1,4-phenylene nitrilomethylidine-1,4-phenylene methylidine), hereafter referred to as PNPM and whose structure is ##STR1## is a polymer which shows many of the aforementioned properties. In particular, PNPM is normally an insulator whose conductivity increases to about 10.sup.-4 ohm.sup.-1 cm.sup.-1 upon doping with an electron acceptor such as iodine. In addition to these properties as a p-type semiconductor, PNPM can be doped with an electron donor such as sodium naphthalide to behave as an n-type semiconductor.
Polyacetylene and poly(p-phenylene) exemplify some better, perhaps the best, prior art electrically conducting polymers, hence their limitations exemplify the prior art constraints. Although polyacetylene may be doped with p- and n-type dopants, all doped as well as undoped polyacetylene is unstable in air. Thus the electrical properties, which are of greatest interest in this application, are useful for only short periods in air. In contrast, poly(p-phenylene) itself is air stable but it affords air unstable, electrically conducting polymers with both p- and n-type dopants. However, these materials invariably are amorphous powders which cannot be cast, hence their processability is severely limited.